Science in Society Archive

Bt is Toxic

Recently there has been considerable international concern about the contamination of the human food chain with StarLink corn containing Baciullus thuriniensis (Bt) toxin Cry 9. Bt Cry 9 toxin had evident allergenicity in test animals, and had been approved for use in animal feed alone, but was found to have contaminated corn and corn products destined for human consumption.

Bt toxins are the products of a number of genes and genes that differ between Bt varieties. The United States and Canada judge that each toxin gene product must be considered safe for human consumption until it is proven otherwise. A study that recently came to light [1] shows that a widely used Bt toxin actually damages the mammalian ileum (the final part of the small intestine, where food stays the longest). Damages to the ileum can produce chronic illness such as fecal incontinence and/or flu like upsets of the digestive system.

The researchers studied the effects of both GM potatoes carrying the CryI gene of Bacillus thuringiensis var. kurstaki strain HD1, as well as non-GM potatoes spiked with the toxin from the same strain of bacterium. Groups of five one-month old male mice were fed daily for 2 weeks on a diet of either the GM potatoes or the non-GM spiked by soaking the diced potatoes for 30 minutes in a suspension of the toxin (1 g per litre). The control group was fed non-GM potatoes for the same duration. Light and electron microscopic structures of the ileum in the three groups were compared.

Both the groups of mice fed GM potatoes or spiked potatoes revealed certain common features such as the abnormal appearance of mitochondria, with signs of degeneration and disrupted short microvilli (microscopic projections on the cell surface) at the surface lining the gut. However, in the group of mice fed on the spiked potatoes, several villi (projections of the intestinal lining, each made up of many cells, not to be confused with microvilli above) appeared with an abnormally large number of cells (151.8 in control group compared to197 and 155.8 in the spiked and GM-fed groups, respectively). Fifty percent of these cells were overgrown with multiple nuclei. The mean area of the cells was significantly increased (105.3 mm2 in control group compared to 165.4 and 116.5 in the spiked and GM-fed groups, respectively).

Several forms of secondary digestive vacuoles were recognised in these cells. These changes were confirmed with the scanning electron microscope which revealed a
remarkable increase in the perimeter of the cells (23 mm in control group compared to 44 and 28 in spiked and GM-fed groups, respectively). The basal lamina along the base of the cells was damaged at several foci. Several disrupted microvilli appeared in association with variable-shaped fragments of broken cells. In addition, the Paneth cells (secretory cells) in the spiked-fed group were highly activated and contained a large number of secretory granules. These changes may suggest that spiked potatoes resulted in the development of hyperplastic (overgrown) cells in the mice ileum.

Although milder changes are reported in the structural configuration of the ileum of mice fed on GM potatoes, the authors recommend that "thorough tests of these new types of genetically engineered crops must be made to avoid the risks before marketing".

It seems clear that the damages to the mice ileum are due to the bt toxin. What is not clear from this paper is the amount of toxin expressed in the GM potatoes compared to the amount in the spiked potatoes.

It is also of interest that the results here are similar to those obtained by Ewen and Pusztai in experiments with GM potatoes expressing the snowdrop lectin. As Pusztai points out, bt is also a lectin. It suggests that all lectins may have detrimental effects on the gut and should never have been used in GM crops.

1. Fares NH and El-Sayed AK. Fine structural changes in the ileum of mice fed on dendotoxin-treated potatotes and transgenic potatoes. Natural Toxins:1998: 6: 219-33.

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